1. Technical Field
The present invention relates to an acceleration sensing device, particularly to an acceleration sensing device in which sensitivity in an acceleration detection axis direction is improved and sensitivities in other axis directions are suppressed.
2. Related Art
Acceleration sensors are widely used in various things ranging from automobiles, aircrafts, and rockets to abnormal vibration monitoring units installed in plants and the like. As an acceleration sensor for household appliances, a Micro Electro Mechanical System (MEMS) sensor whose acceleration sensing structure is fabricated by using a semiconductor process technique has been commonly known.
JP-A-2006-64397 is an example of related art. The example discloses an acceleration sensing element. FIG. 5A is a plan view of a known acceleration sensing element 81. The acceleration sensing element 81 has a base part 82, a pair of drive vibrating parts 83, 84 that protrude out from edges of the base part 82 in parallel each other, and a sensing vibrating part 85. The drive vibrating parts 83, 84 and the sensing vibrating part 85 are long and narrow flexural vibrating arms. The drive vibrating part 83 has a pair of grooves 86a, 86b (not shown in the drawing) on its upper and lower faces respectively. An electrode 88 is provided inside and on the sidewall of the grooves 86a, 86b. The electrode 88 is an electrode for exciting the drive vibrating part 83 in such a way indicated by the arrow “A” shown in FIG. 5B. The drive vibrating part 83 includes a strip-shaped main portion and a wide portion 96 at its end part.
The other drive vibrating part 84 has a strip and plate like shape. An electrode 87 is disposed on the surface of the drive vibrating part 84. The electrode 87 is an electrode for exciting the drive vibrating part 84 in such a way indicated by the arrow “A” shown in FIG. 5B. The strip-and-plate shaped sensing vibrating part 85 has a pair of grooves 89a, 89b (not shown in the drawing) on its upper and lower faces respectively. An electrode 90 is provided in the grooves 89a, 89b and on a side wall of the vibration part. The electrode 90 is an electrode through which it is possible to detect vibration of the sensing vibrating part 85 in the direction indicated by the arrow “C” in the drawing.
Referring to FIG. 5B, the drive vibrating parts 83, 84 in the acceleration sensing element 81 vibrate flexuously and in the phase opposite each other as indicated by the arrow “A”. Moreover, vibration frequencies of the drive vibrating parts 83, 84 at the time of self-excited oscillation are made the same, which means that an amplitude of the sensing vibrating part 85 disposed at the center becomes zero when no acceleration is applied.
When acceleration is applied to the acceleration sensing element 81 in the direction indicated by the arrow “B” in FIG. 5B, a force is applied to the drive vibrating parts 83, 84 in an X axis direction. The drive vibrating parts 83, 84 thereby extend in the X axis direction and their vibration frequencies both increase. The frequency change (increase) of the drive vibrating part 83 is larger than the frequency change (increase) of the drive vibrating part 84 since the drive vibrating part 83 has the wide portion 96 that holds a large weight at its end. Consequently moments “mα” (m: mass, α: acceleration) of the drive vibrating parts differ each other, and which generates a flexural vibration of the sensing vibrating part 85 in a Y axis direction as denoted by the arrow “C” shown in FIG. 5B. The example describes that an amplitude of the flexural vibration “C” monotonically increases as the acceleration “B” increases and the amplitude is substantially proportional to an output from a detection electrode of the sensing vibrating part 85 thereby it is possible to obtain the acceleration “B”.
However the acceleration sensing element disclosed in the example made one of the drive vibrating parts have a larger weight compared to that of the other vibration part in order to unbalance the vibration system. For this reason, it is difficult to realize both a small sized sensing element and to improve the sensitivity.
Moreover the acceleration sensing element disclosed by the example has a disadvantage of sensitivity in an unintended axis. The sensing element of the example has more than one sensible acceleration axis (the X axis direction in FIG. 7) with which the vibration system is unbalanced, thereby acceleration in other direction is also detected when the acceleration is applied to for example the orthogonal direction to the drive vibrating part.